The Pinto shear zone; a Laramide synconvergent extensional...
Transcript of The Pinto shear zone; a Laramide synconvergent extensional...
The Pinto shear zone; a Laramide synconvergent extensional shear zone in
the Mojave Desert region of the southwestern United States
Michael L. Wellsa,*, Mengesha A. Beyenea, Terry L. Spella, Joseph L. Kulaa, David M. Millerb,
Kathleen A. Zanettia
aDepartment of Geoscience, University of Nevada Las Vegas, Las Vegas, NV 89154, USAbU.S. Geological Survey, MS-973, 345 Middlefield Road, Menlo Park, CA 94025, USA
Received 6 April 2004; received in revised form 29 September 2004; accepted 11 March 2005
Available online 15 July 2005
Abstract
The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest
Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional
origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in
deformation temperature, and differences in hanging wall and footwall thermal histories. Deformation is constrained between w74 and
68 Ma by 40Ar/39Ar thermochronology of the exhumed footwall, including multi-diffusion domain modeling of K-feldspar. We discount the
interpretations, applied in other areas of the Mojave Desert region, that widespread Late Cretaceous cooling results from refrigeration due to
subduction of a shallowly dipping Laramide slab or to erosional denudation, and suggest alternatively that post-intrusion cooling and
exhumation by extensional structures are recorded. Widespread crustal melting and magmatism followed by extension and cooling in the
Late Cretaceous are most consistent with production of a low-viscosity lower crust during anatexis and/or delamination of mantle lithosphere
at the onset of Laramide shallow subduction.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Laramide tectonics; Synconvergent extension; Delamination; Mojave Desert
1. Introduction
The Mesozoic to early Cenozoic Sevier–Laramide
orogens of the western US—segments of the larger
Cordilleran orogen (Burchfiel et al., 1992)—are classic
examples of backarc belts of crustal shortening, considered
to be ancient and more deeply exhumed analogues to the
modern Andean orogen (Burchfiel and Davis, 1976; Jordan
et al., 1983; Isacks, 1988). Post-orogenic Cenozoic
extension of the hinterland of the Sevier–Laramide orogen
is well-recognized (Wernicke, 1992 and references therein)
whereas older, synorogenic Cretaceous extension is more
cryptic. Cretaceous extension has been documented in
scattered mountain ranges of the Great Basin (Wells et al.,
1990, 1998; Camilleri and Chamberlain, 1997) and Mojave
0191-8141/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsg.2005.03.005
* Corresponding author. Tel.: C1 7028953262; fax: C1 7028954064
E-mail address: [email protected] (M.L. Wells).
Desert regions (Carl et al., 1991; Applegate et al., 1992;
Beyene et al., 2000; Wells et al., 2002) and was possibly an
orogen-scale event (Hodges and Walker, 1992) (Fig. 1).
Late Cretaceous extension was synchronous with continued
convergence between the Farallon/Kula plates and western
North America (Engebretson et al., 1985), continued
shortening in the fold–thrust belt north of southern Utah
(Decelles et al., 1995; Yonkee et al., 1997) and early
shortening in the Laramide foreland province (e.g.,
Dickinson et al., 1988) (Fig. 1). Although prior crustal
thickening during Sevier orogenesis undoubtedly played an
important role in providing requisite lateral contrasts in
crustal thickness to gravitationally drive Cretaceous
extension, it remains unclear what changes in the dynamics
of the orogen took place to trigger synconvergent extension.
A change is required, otherwise compressional forces
sufficient to initially thicken the crust to form an orogenic
plateau would continue to support the plateau against
gravitationally induced extension (Molnar and Lyon-Caen,
1988). To better understand the mechanisms triggering
Journal of Structural Geology 27 (2005) 1697–1720
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Fig. 1. Simplified tectonic map of the western Cordillera showing selected
Cretaceous to early Tertiary features and location of study area. Belt of
muscovite granites (grey fill; Miller and Bradfish, 1980) largely coincides
with the belt of metamorphic core complexes (black fill) and inferred axis
of maximum crustal thickening in the Cordilleran orogen. The leading edge
of Cordilleran fold–thrust belt (bold lines with teeth) converges with the
magmatic arc (cross pattern), towards the south in southeastern California.
Box shows location of the study area (Fig. 2) in the eastern Mojave Desert.
F, Funeral Mountains; P, Panamint Range.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201698
Cretaceous extension in the southwestern Cordillera, it is
important to establish whether the onset of extension was
regionally synchronous, diachronous, or localized at any
one time, and also whether there is a consistent relationship
between the onset of extension and magmatism. Resolving
these issues requires better age constraints on the inception
and duration of extension, clear distinctions between
extensional and contractional origins for structures, and
additional geochronological studies of Cretaceous plutons.
The Cordillera of the southwest US is particularly well
suited to establish better age constraints for Mesozoic
deformation, and Cretaceous extension in particular,
because the eastern fringe of the Mesozoic magmatic arc
interacts with both contractional and extensional structures.
For example, in the Old Woman Mountains area,
thermochronometry and geobarometry of Cretaceous
plutons shows rapid cooling from 73 to 68 Ma due to
exhumation by syn- to post-magmatic extensional shear
zones following thrust burial to mid-crustal depths (Carl et
al., 1991; Foster et al., 1992; McKaffrey et al., 1999)
(Fig. 2). Structural unroofing of metamorphic rocks in the
Funeral Mountains of the Death Valley region (Fig. 1)
occurred during intrusion of Late Cretaceous dikes and sills
at 72 Ma (Hodges and Walker, 1990; Applegate et al., 1992;
Applegate and Hodges, 1995). K–Ar and 40Ar/39Ar cooling
ages in other areas of the Mojave desert (Evernden and
Kistler, 1970; Armstrong and Suppe, 1973; Kistler and
Peterman, 1978; Miller and Morton, 1980; Jacobson, 1990;
Foster et al., 1990) and southern Sierra (Wood and Saleeby,
1997) suggest the possibility of regional extension at 75–
68 Ma. However, alternative mechanisms for cooling,
including refrigeration (Dumitru et al., 1991) and uplift-
induced erosion (George and Dokka, 1994) resulting from
subduction of a shallowly dipping buoyant Laramide slab
must also be considered.
In this paper, we summarize our new geologic mapping,
structural data, and 40Ar/39Ar thermochronometric studies
of the Pinto shear zone in the New York Mountains of the
northeastern Mojave Desert (Figs. 2 and 3). Previous
workers have speculated that the Pinto shear zone is a
Mesozoic thrust (Beckerman et al., 1982) or alternatively a
Late Cretaceous normal fault (Miller et al., 1996); this
discrepancy in part reflects the difficulty of distinguishing
between extensional and contractional shear zones. To make
this distinction and to resolve contrasting interpretations, we
bring to bear a broad array of observations, including shear
zone geometry and kinematics, hanging wall deformation
style, progressive changes in deformation temperature, and
differences in hanging wall and footwall thermal histories.
From these data we conclude that the Pinto shear zone is an
extensional shear zone of Late Cretaceous age. This shear
zone lies intermediate between areas to the north and south
in which latest Cretaceous extension has been previously
documented. Contrary to the interpretation that 74–68 Ma
cooling ages result from refrigeration of the Cordilleran
lithosphere above a shallowly subducting Laramide slab
(Dumitru et al., 1991) or erosional denudation (as suggested
for the Peninsular Ranges; George and Dokka, 1994) we
suggest that many of the Late Cretaceous cooling ages in the
eastern Mojave Desert region result from post-intrusive
cooling and exhumation by extensional structures. Late
Cretaceous extension at 75–68 Ma was regional in scale in
the southwestern Cordillera, and therefore requires a
regional causative mechanism. We propose that Late
Cretaceous extension of the Mojave sector of the
Cordilleran orogen is most consistent with production of a
low-viscosity lower crust during anatexis and/or removal of
mantle lithosphere at the onset of Laramide shallow
subduction.
2. Southern Cordilleran Mesozoic arc and thrust belt
The eastern Mojave Desert region is located at the site of
intersection between the Mesozoic Cordilleran fold–thrust
belt and the eastern fringe of the partly coeval, composite
magmatic arc (Figs. 1 and 2). The magmatic arc rocks of the
region include Jurassic plutonic and volcanic rocks (Glazner
et al., 1994; Schermer and Busby, 1994; Fox and Miller,
1995; Gerber et al., 1995; Walker et al., 1995),
Fig. 2. Simplified geologic map of the eastern Mojave Desert of California and southern Nevada, showing location of the Pinto shear zone in the southern New
York Mountains. The study area lies within the southern extension of the Clark Mountain thrust complex within the southern Cordilleran fold–thrust belt.
Deformed roof rocks to the mid-Cretaceous Teutonia batholith display structures intermediate between thin-skinned styles to the north (eastern Clark Mountain
thrust complex) and basement-cored nappes to the south (Old Woman and Big Maria Mountains). CM, Clipper Mountains; PM, Piute Mountains; KH, Kilbeck
Hills; LMM, Little Maria Mountains; BMM, Big Maria Mountains. RC, rapid cooling; SZ, extensional shear zone activity; numbers in million years (Ma). Box
shows location of Fig. 3. Modified from Jennings (1977), Howard et al. (1987), Miller et al. (1991) and Anderson et al. (1992).
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1699
mid-Cretaceous granitoids and minor volcanic rocks, and
abundant Late Cretaceous metaluminous and peraluminous
plutons (Armstrong and Suppe, 1973; Burchfiel and Davis,
1981; Beckerman et al., 1982; Foster et al., 1990; John and
Mukusa, 1990; Fleck et al., 1994). The Teutonia batholith is
one of the larger intrusive complexes in the eastern Mojave
Desert (Figs. 2 and 3) and is comprised of 90–97 Ma
metaluminous to weakly peraluminous granitic plutons
(Beckerman et al., 1982; Anderson et al., 1992).
The Cordilleran fold–thrust belt is discontinuously
preserved between the southernmost region of thin-skinned
thrusting in the Clark Mountain thrust complex (Burchfiel
and Davis, 1971) and the basement-involved fold nappes of
the Big Maria and Dome Rock Mountains (Hamilton, 1987;
Boettcher and Mosher, 1998) of the Maria tectonic belt
(Reynolds et al., 1986) (Fig. 2). Ductile fold nappes and
thrusts within the Kilbeck Hills, Old Woman–Piute
Mountains, and Clipper Mountains (Fig. 2) suggest
substantial localized structural burial during the Mesozoic
(Howard et al., 1987; Horinga, 1988; Fletcher and
Karlstrom, 1990; Foster et al., 1992; Howard et al., 1995)
and permit linking of the northern and southern contractile
belts (Burchfiel and Davis, 1981).
3. Geology of the southern New York mountains
Phanerozoic rocks are preserved along the eastern
Fig. 3. General geology of the Teutonia batholith and surrounding region. Note that Mesozoic structures of the Clark Mountain thrust complex correlate across
the Ivanpah Valley to the New York Mountains: the Kokoweef fault to the Slaughterhouse fault, and the Keaney–Mullusk Mine thrust to the Sagamore Canyon
thrust in the New York Mountains (Burchfiel and Davis, 1977; Smith et al., 2003). The Pinto shear zone lies in the hanging wall of the thrust stack exposed in
the southeastern New York Mountains. Box shows location of Fig. 4. Modified from Beckerman et al. (1982) and Miller and Wooden (1993).
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201700
margin and within the roof of the Teutonia batholith in the
southern New York Mountains, about 25 km southeast of
the southern extent of the Clark Mountain thrust complex in
the Mescal Range (Burchfiel and Davis, 1977; Beckerman
et al., 1982) (Figs. 2–4). Substantial Mesozoic crustal
thickening and attributes of both the northern and southern
structural styles are evident; a stack of thrust duplications
include Paleozoic over Mesozoic rocks, and Precambrian
crystalline rocks are involved in map-scale folds (Burchfiel
and Davis, 1977). The youngest low-angle fault that is
clearly a thrust cuts 100 Ma metavolcanic rocks equivalent
in age to the Delfonte volcanic rocks of the Mescal Range
(Fleck et al., 1994; Smith et al., 2003) and predates intrusion
of the Mid Hills monzogranite. These timing constraints are
similar for the frontal thrust in the Clark Mountain thrust
complex (Fleck et al., 1994; Walker et al., 1995).
The Teutonia batholith, as originally defined by Becker-
man et al. (1982), is comprised primarily of early Late
Cretaceous metaluminous to weakly peraluminous granitic
plutons with lesser Late Jurassic granitic rocks; a more
restricted usage to include only Cretaceous intrusions was
introduced by Anderson et al. (1992) and is adopted here.
Several geochronological investigations have clarified the
ages for some of the plutonic phases of the batholith but
remaining ambiguities warrant additional work (Sutter,
1968; Beckerman et al., 1982; Dewitt et al., 1984; Walker et
al., 1995; Miller et al., 1996). Two phases of the Teutonia
batholith crop out in the New York Mountains: the Mid
Hills adamellite of Beckerman et al. (1982), here called the
Mid Hills monzogranite, and the Live Oak Canyon
granodiorite of Beckerman et al. (1982) (Miller and
Wooden, 1993). The Mid Hills monzogranite crops out
over an area O300 km2 that extends from the eastern New
York Mountains to the southern Mid Hills (Fig. 3), and
intrudes Precambrian gneiss along its southern margin and
both Paleozoic and Mesozoic metasedimentary rocks along
its eastern margin. The Mid Hills monzogranite varies in
mineralogy and texture and is probably a composite pluton.
Fig. 4. Geologic and sample location map of the Pinto Valley area of the central New York Mountains, simplified from Miller et al. (1991), Burchfiel and Davis (1977), Beyene (2000) and Smith et al. (2003).
Note overall Z-shaped map pattern to the Pinto shear zone and division of shear zone into six geometric structural domains (numbers on map, dashed lines represent domain boundaries), with corresponding
stereograms of lineations (smaller circles), poles to foliation (larger boxes), and average foliation (dashed great circle). 40Ar/39Ar sample localities for samples reported here are shown, and U–Pb geochronology
sample localities of Smith et al. (2003). The absence of mapped dikes in the central part of the map may reflect lack of detailed mapping. Area outlined by short dashed line in hanging wall above domains 3–5
represents study area of porphyry dikes. Inset: cross-section A–A0 along western thermochronology sampling transect, showing geometry of the shear zone and sample locations. Cross-section is of larger scale
than map.
M.L.Wells
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M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201702
In Round Valley (Fig. 3), it has been dated at about 92 Ma
by U–Pb on zircon (Miller et al., 1996), and recently at
88.2C1.6 Ma (1 sigma) by U–Pb SHRIMP on zircon (Barth
et al., 2004). Smith et al. (2003) report an ion probe U–Pb
zircon date of 90.0G1.3 Ma (1 sigma) on the Mid Hills
monzogranite in Pinto Valley (Fig. 4, location NY-1).
Numerous NE- to E-striking granodiorite porphyry dikes
cut the Teutonia batholith and its country rocks and are
more abundant near its roof and walls (Miller et al., 1996)
(Fig. 4). The porphyry dikes in the New York Mountains
have a distinctive texture and mineralogy including zoned
K-feldspar, embayed quartz phenocrysts, and biotite in a
fine-grained groundmass suggesting hypabyssal emplace-
ment. Smith et al. (2003) report a U–Pb zircon date of
79.7G3.6 Ma (1 sigma) for a porphyry dike south of
Sagamore Canyon (Fig. 3; location NY-11), determined by
TIMS on multigrain fractions. Recent ion microprobe
analyses of zircon from this dike sample (Wells, unpub-
lished data) indicate the presence of multiple inherited
zircon fractions from older phases of the Teutonia batholith
and a younger age of 74.6G3.2 Ma for magmatic rims. This
revised age is consistent with field relations; the dikes cut a
pegmatite previously dated at 76–77 Ma by U–Pb zircon
(Miller et al., 1996), in the Live Oak Canyon area of the
New York Mountains.
4. The Pinto Shear zone
A mylonite zone first identified by Beckerman et al.
(1982) and subsequently named the Pinto shear zone by
Miller et al. (1996) crops out along the north side of Pinto
Valley and continues northwest over the crest of the New
York Mountains and into Ivanpah Valley (Figs. 3 and 4).
The shear zone deforms the Mid Hills monzogranite and
younger dikes and is truncated on the southeast by the
Cenozoic Cedar Canyon fault and inferred to be truncated
on the northwest by the Nipton fault in the subsurface of
Ivanpah Valley (Miller et al., 1991, 1996). The shear zone
strikes NW overall, but in detail has a Z-shaped map pattern
(Fig. 4). With a thickness of about 550–600 m where
mylonitic in the central part, the shear zone is thinner
towards the north and ultimately grades northward into a 20-
m-wide breccia zone; the brittle fault zone may have excised
mylonitic rocks along the northern reaches of the shear
zone. Deformation fabric in the central part generally
increases structurally upward defining a strain gradient from
undeformed granitoid in the footwall, through protomylo-
nite, mylonite, to ultramylonite of variable thickness of 5–
40 m at the top of the shear zone. Decameter-scale lenses of
lesser-deformed rock that deviate from the strain gradient,
however, are present within the shear zone. A brittle fault
zone overprints the shear zone at variable levels near its top
within the ultramylonite. Mylonite and ultramylonite zones
are also developed along the margins of porphyry dikes in
the hanging wall.
4.1. Kinematics and kinematic model
Kinematic studies were conducted on the Mid Hills
monzogranite and porphyry dikes within the main shear
zone and on discrete shear zones within the hanging wall, of
which most are localized on the margins of porphyry dikes.
Although sinuous in map pattern, foliation within the shear
zone generally dips 20–658S and SW, and lineation plunges
SSW (Fig. 4). Kinematic studies of the Mid Hills
monzogranite within the Pinto shear zone consistently
yield non-coaxial top-to-the-SSW shear. Shear sense
indicators are abundant, and all indicators including S–C
fabrics (Berthe et al., 1979; Lister and Snoke, 1984), mica
fish (Lister and Snoke, 1984), winged feldspar porphyr-
oclasts (Passchier and Simpson, 1986), myrmekite quarter
structures (Simpson and Wintsch, 1989), C 0-type shear
bands (Passchier and Trouw, 1996), and oblique dynami-
cally recrystallized quartz grain-shape (Law et al., 1984;
Lister and Snoke, 1984) show top-to-the-SSW sense of
shear (Fig. 5(a)–(d)).
The deformation of porphyry dikes varies across the
Pinto shear zone and imposes important constraints on its
kinematic evolution. A geometric and kinematic analysis of
the dikes was conducted in an area of the hanging wall west
of structural domains 3–5 of the shear zone (Fig. 4). The
majority of porphyry dikes within the hanging wall are E-
and NE-striking (Fig. 4), steeply N-dipping (40–758) and
exhibit localized mylonite to ultramylonite along their
margins. A solid-state foliation is subparallel to the dike
margins, and lineation is generally down-dip and varies
from 3128/638 (trend/plunge) to 3508/398 (Fig. 6a). Sense-
of-shear indicators (Fig. 5e and f), including ubiquitous
biotite fish, record a consistent down-to-the-NNW shear,
antithetic to the shear in the rest of the Pinto shear zone. A
few dikes are N-striking and steeply dipping; these contain
shallowly plunging lineation and record dextral shear
(Fig. 6a). Several porphyry dikes in the hanging wall can
be traced downwards across the upper shear zone boundary
and into ultramylonitic rocks. The dikes are progressively
thinned and deflected towards parallelism with c-surfaces of
the ultramylonite as they cross the shear zone downwards,
and a transition from down-to-the-NNW to top-to-the-SW
shear is noted with increasing deformation of the dike.
Overprinting relationships between these oppositely
directed fabrics are not apparent.
Within the shear zone (domains 3–5), the porphyry dikes
are of variable orientation and accommodate both synthetic
and antithetic shear relative to the main shear zone (Figs. 6b
and 7). Within the central part of the shear zone, dikes
striking E to NNE with a component of north dip, exhibit a
solid-state foliation throughout their widths and a north
trending lineation. Kinematic studies of the dikes show
antithetic down-to-the-N shear throughout their widths and,
weak to moderately developed SW-dipping foliation within
the monzogranitic country rock shows an apparent
deflection into parallelism with the dike margin as the
Fig. 5. Kinematic indicators from the Pinto shear zone ((a)–(d)), and from the antithetically-sheared porphyry dikes ((e) and (f)). (a) Oblique grain-shape fabric
in dynamically recrystallized quartz. SA is defined by mica-rich zones of concentrated strain parallel to quartz ribbons and SB is defined by the long axis of
dynamically recrystallized quartz grains (Law et al., 1984). (b) Biotite (bt) fish from deformed porphyry dike in the central part of the shear zone. (c) s-Type K-
feldspar porphyroclast showing top-to-the-SW (sinistral) sense of shear. (d) Deformed quartz vein shows elongate quartz ribbons (SA) and oblique grain-shape
fabric (SB) of dynamically recrystallized quartz. (e) Synthetic rotation resulting from antithetic microfracturing along the cleavage of plagioclase feldspar in
porphyry dike, indicating top-to-the-N (dextral) sense of shear. Note synthetic shearing of biotite fish along basal cleavage. (f) Mica fish in porphyry dike,
indicating dextral (top-to-the-N) sense of shear. Scale bar, one millimeter.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1703
Fig. 6. Kinematic analysis of porphyry dikes. Foliation tends to parallel the
dike margin and serves as a proxy for dike orientation. (a) Foliation and
lineation in porphyry dikes of the hanging wall adjacent to domains 4 and 5
of shear zone. Note two orientation populations: (1) the predominant NE- to
NW-striking, N-dipping dikes, and (2) the subordinate N-striking, sub-
vertical dikes. North-dipping dikes show antithetic top-to-the-N shear, and
N-striking dikes show dextral shear. (b) Foliation and lineation within
porphyry dikes within the shear zone. Dikes are in three principal
orientations: (1) E-striking and north dipping, (2) E-striking and south
dipping, and (3) N-striking and west dipping. S-dipping dikes show top-to-
the-SW shear (synthetic), N-dipping dikes show antithetic top-to-the-N
shear, and N-striking dikes show dextral-normal shear. (c) and (d) ‘P’
(infinitesimal shortening, filled circles) and ‘T’ (infinitesimal extension,
open squares) axes determined for antithetic shears along N-dipping dike
margins; contours are of ‘T’ axes by Kamb method.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201704
strain intensity increases, compatible with the determined
shear sense (Fig. 7). The dikes are progressively rotated with
increasing amount of antithetic shear within the dike and
increasing amount of synthetic shear in the country rock.
Where the dikes are W- to NW-striking with a component of
south dip, typically within more highly strained parts of the
shear zone, they record top-to-the-SW sense of shear
synthetic to the shear in the surrounding mylonitic
monzogranite. In general, the lineation in the dikes within
the shear zone have similar trend to the local monzogranitic
mylonite (compare Figs. 4 and 6b). Porphyry dikes in the
footwall are undeformed, and dikes cannot be continuously
mapped across the shear zone to assess total displacement.
Two observations are critical to deciphering the relative
timing between down-to-the-N shear along the dike margins
and top-to-the-S shear within the Pinto shear zone. (1) The
porphyry dikes in the hanging wall exhibit down-to-the-N
kinematics and are progressively deformed and rotated
downwards into the ultramylonite zone, and where
significantly thinned and rotated, exhibit top-to-the-S shear:
continuations of zones of down-to-the-N shear through the
ultramylonite are not apparent (Fig. 7). (2) Foliation in lesser-
deformed lenses in Mid Hills monzogranite within the shear
zone show abrupt increases in intensity in concert with
progressive rotation into parallelism with the margin of
N-dipping dikes that exhibit antithetic shear throughout their
widths (Fig. 7). The simplest interpretation that reconciles
these two relationships of relative timing is that the antithetic
high-angle normal sense shears were broadly active simul-
taneously with top-to-the-S shear along the main zone.
The similarity in azimuths of movement direction
inferred from the lineation within the dike-margin shears
in the hanging wall, deformed porphyry dikes within the
shear zone, and the mylonite and ultramylonite derived from
monzogranite within the shear zone support this view by
suggesting kinematic compatibility (Figs. 4 and 6). This can
be further appreciated by consideration of the similarity in
best-fit kinematic axes of infinitesimal extension for the
antithetic shearing in the hanging wall (Fig. 6c) to those
within the shear zone (Fig. 6d), determined using the fault-
slip kinematic analysis approach of Marrett and Allmen-
dinger (1990) (FaultKin 4.1X; Allmendinger, 2001).
Internal rotation of hanging wall blocks synthetic to the
main shear zone was accomplished by synchronous
antithetic slip along high-angle normal shears, which were
influenced by the orientation and spacing of the dikes
(Fig. 7). Dike margins in the hanging wall along the range
crest 3 km to the west, and along the mountain flank on the
east side of Ivanpah Valley, including some of the same
continuous dikes that show down-to-the-N shear closer to
the roof of the shear zone, do not exhibit solid-state foliation
at their margins. As dictated by strain compatibility, the
shear must terminate at an upper boundary analogous to a
kink band, separating rocks that rotated during shearing
from rocks that did not (Fig. 7). The kinematics of synthetic
block rotation both within the shear zone and the hanging
wall was influenced by the orientation of the dikes. The
localization of antithetic shearing along the dike margins
suggests that the porphyry dikes, with their fine-grain
groundmass, represented weak zones along which defor-
mation was localized. The anisotropy due to their systematic
orientation controlled the style of synthetic block rotation. A
similar geometry is described within a Miocene extensional
shear zone in the Sacramento Mountains of the Colorado
River trough region (Pease and Argent, 1999; Stewart and
Argent, 2000). In the Sacramento Mountains, antithetic
shear, localized along dikes, accommodated synthetic
rotation of blocks.
It is interpreted that the porphyry dikes intruded along
parallel, or perhaps en-echelon extension fractures during
generally N–S extension within the Teutonia batholith and
its wall rocks. The U–Pb age of 74.6G3.2 Ma for the
porphyry dikes permits intrusion to have occurred prior to or
during early movement along the shear zone. While the fine-
grained groundmass of the dikes suggests hypabyssal
Fig. 7. Schematic model showing deformation style of the Mid Hills monzogranite and porphyry dikes within the Pinto shear zone and hanging wall. Steep
shear zones antithetic to the principal shear sense are localized along porphyry dikes and bound blocks that experienced rotation synthetic to the principal shear
sense, requiring an upper kink-band boundary. Lesser-deformed lenses within the shear zone experienced thinning by antithetic motion along shear zones
localized within porphyry dikes. Inset in lower right shows view of the margin of an E-striking, N-dipping porphyry dike within the shear zone, looking W.
Dashed line is dike margin, solid lines are form lines of foliation.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1705
emplacement, depth estimates independent of the cooling
histories are not available to allow the depth of the Mid Hills
monzogranite, prior to dike emplacement, to be assessed.
We note that the dikes strike WNW where mapped in the
footwall of the Pinto shear zone in the Live Oak Canyon
area, but strike NE in the hanging wall (Fig. 4); while this
difference in orientation is probably original, it cannot be
ruled out that a component of vertical axis rotation was
accomplished along the shear zone.
4.2. Deformation mechanisms
Study of deformation mechanisms from microstructures
within the Mid Hills monzogranite show an apparent
decrease in temperature during progressive shearing, from
plastic deformation at upper greenschist to lower amphibo-
lite facies conditions to brittle deformation conditions.
K-feldspar displays abundant evidence for early dynamic
recrystallization followed by later cataclasis. K-feldspar is
dynamically recrystallized at porphyroclast tails (Fig. 8a–c),
which extend into layers defining mylonitic foliation
(Fig. 8a). Strain-induced myrmekite (quartzCplagioclase)
growth is ubiquitous on K-feldspar grain boundaries that are
facing the inferred incremental shortening direction, and
dynamic recrystallization of myrmekite was apparently an
important matrix-producing mechanism as bands of micron-
sized quartzCplagioclase alternate in the matrix with quartz
ribbons and bands of dynamically recrystallized K-feldspar
(Fig. 8b and c). New K-feldspar is locally grown in dilatant
sites including micro pull-aparts and strain shadows. These
microstructures and mineral reactions most probably occur
in K-feldspar in the temperature range of 450–550 8C
(Simpson and Wintsch, 1989; FitzGerald and Stunitz, 1993;
Pryer, 1993; Passchier and Trouw, 1996). Deformation
continued at somewhat lower temperature conditions as
indicated by flame perthite, undulose extinction, and kink
bands in K-feldspar (w350–450 8C) (Pryer, 1993). Tensile
fractures cut K-feldspar cores, dynamically recrystallized
new grains and myrmekite, and are infilled with muscov-
iteCquartz (Fig. 8c). The brittle deformation overprint on
early plastic features is greatest near the top of the shear
zone, and grain size reduction by fracture suggests
temperatures !350 8C (G50). Where Paleozoic carbonate
is present in the immediate hanging wall, extensive
secondary retrograde alteration and mineralization
accompanied cataclasis, and fine-grained epidote and
chlorite filled and sealed fractures and cataclastic crush
zones of fine-grained angular fragments, and calcite locally
replaced plagioclase (Fig. 8d).
Quartz also shows evidence for dynamic recrystallization
during decreasing temperature conditions. At lower
structural levels in the shear zone completely recrystallized
quartz ribbons (Type 2a of Boullier and Bouchez, 1975)
with interlobate grain boundaries indicate dominantly high-
temperature grain boundary migration recrystallization
(Stipp et al., 2002). These grains are locally overprinted
by pervasive low-T plasticity including deformation
lamellae. In other samples, particularly at higher structural
levels within the shear zone, progressive development of
subgrains and new grains within and around larger quartz
Fig. 8. Microstructures illustrating deformation mechanisms in the Mid Hills monzogranite. (a) Sigma-shaped plagioclase (Pl) porphyroclast with mantle
(upper margin) and tails of fine-grained dynamically recrystallized grains (dyn Pl). Tail of K-feldspar porphyroclast in upper right is dynamically recrystallized
and forms more coarse grained band (dyn Kfs) extending to left. (b) K-feldspar porphyroclast with extensive myrmekite (Myr) formation on upper right and
lower left margins. Note banding defined by alternating layers of quartz ribbons (Qtz) and fine-grained feldspar and quartz. Textural relations suggest that
myrmekite sited on K-feldspar porphyroclasts is progressively dynamically recrystallized and forms fine-grained bands in matrix. (c) K-feldspar porphyroclast
with extension fractures that cut clast, marginal myrmekite, and dynamically recrystallized K-feldspar in tails. Fractures are filled by muscoviteCquartz. (d)
Fault rock from brittle fault capping ultramylonite. Fine-grained epidote and chlorite fill and seal fractures and cataclastic crush zones of fine-grained angular
fragments. Scale bar, one millimeter.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201706
ribbons suggests subgrain-rotation recrystallization correla-
tive to regime 2 of Hirth and Tullis (1992) (Stipp et al.,
2002) (Fig. 5d). Quartz ribbons are cut by extension
fractures filled by muscovite and quartz, indicating a
superposition of brittle (!300 8C) on plastic fabrics.
5. 40Ar/39Ar Thermochronology
Seven samples were collected from the footwall, hanging
wall, and within the shear zone for 40Ar/39Ar thermochronol-
ogy along two transects, to be referred to as the eastern and
western transects (Fig. 4). In total, five muscovites, three
biotites, and four K-feldspars were analyzed. Sample locations
are shown in Fig. 4. A description of the laboratory procedures
(Appendix A), the 40Ar/39Ar data tables (Appendix B), Ca/K
plots (Appendix C) and isochron plots for micas (Appendix D)
are available as electronic supplements. Results are presented
in Figs. 9–12. Data are presented at the one-sigma level of
uncertainty.
5.1. Muscovite and biotite
5.1.1. Western transect
Muscovite in the three samples of Mid Hills monzogranite
occurs dominantly as secondary (hydrothermal or meta-
morphic) grains in variably oriented microscopic fractures
within mineral grains of the monzogranite, with lesser
magmatic muscovite. Muscovite (NY-1M) from the hanging
wall of the Pinto shear zone (Fig. 4) shows a stair-stepped age
spectrum with ages increasing from 68.0G0.4 to 74.4G0.4 Ma with increasing temperature and a total gas age of
72.0G0.4 Ma (Fig. 9a). Coexisting biotite (NY-1B) yields a
discordant age spectrum with a slight saddle at intermediate
temperature steps, and a total gas age of 71.2G0.4 Ma.
Muscovite (NY-25M) was analyzed from a quartz vein
Fig. 9. The 40Ar/39Ar apparent age spectra of muscovite (M) and biotite (B) from the Pinto Valley area. (a) Muscovite and biotite (NY-1, Mid Hills
monzogranite) from the hanging wall of the Pinto shear zone along the western transect. (b) Muscovite (NY-26, Mid Hills monzogranite) and biotite (NY-40,
porphyry dike) from the footwall and muscovite (quartz vein) from within the shear zone from the western transect. (c) Muscovite and biotite from the eastern
transect. (d) Comparison between muscovite from aplite and quartz vein within the shear zone. (e) Comparison between muscovite from the Mid Hills
monzogranite. (f) Comparison between biotite from the Mid Hills monzogranite.
Fig. 10. The 40Ar/39Ar apparent age spectra of K-feldspar. Modeled age spectra, following the approach outlined in the text and Fig. 11, are shown in grey scale.
Note excellent fit between model and measured age spectra.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1707
Fig. 11. Multiple diffusion domain modeling of K-feldspar from the Mid Hills monzogranite. Example shown is NY-2 from the footwall of the Pinto shear
zone; all samples produced equally good agreement between modeled and experimentally derived (a) Arrhenius parameters and (b) logr/ro plots. (d) Fifty
cooling histories determined from 10 E–Do pairs. The distribution of the 50 calculated cooling histories for each sample reflects the uncertainty in the obtained
activation energies. Model age spectra (c) are produced by cooling histories (d) shown.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201708
within the shear zone (Fig. 4). This quartz vein is weakly
deformed and exhibits large quartz grains with serrate grain
boundaries leading to the development of new smaller
grains by grain boundary migration recrystallization, a
bimodal grain size and a lattice-preferred orientation.
Muscovite occurs in grain clusters and has a weak preferred
orientation. This sample produced a flat age spectrum, with
a plateau age of 71.9G0.3 Ma (Fig. 9b), indistinguishable
from the total gas age of 71.9G0.4 Ma and isochron age of
72.0G0.8 Ma (MSWDZ1.7).
Muscovite (NY-26M) from the Mid Hills monzogranite
in the footwall of the Pinto shear zone (Fig. 4), similar to
NY-1M, occurs dominantly as secondary grains with lesser
magmatic muscovite. The sample yielded a slight age
gradient from 68.9G0.6 to 72.4G0.6 Ma (Fig. 9b) and a
total gas age of 70.4G0.4 Ma. Biotite lacking chemical
alteration and suitable for analysis from the footwall in the
western transect was only found in an undeformed porphyry
dike that intrudes the Mid Hills monzogranite. Biotite (NY-
40B) shows a flat age spectrum with a plateau age of 71.0G0.4 Ma, indistinguishable from the total gas age of 70.9G0.4 Ma (Fig. 9b).
5.1.2. Eastern transect
Cenozoic sedimentary and volcanic rocks cover the
hanging wall in the southeastern part of the shear zone.
Samples were collected from within the shear zone and from
the undeformed footwall.
Muscovite (NY-49M) from a mylonitic alaskite dike
within the shear zone (Fig. 4) yields a flat age spectrum and
plateau age of 72.3G0.3 Ma (Fig. 9c), a total gas age of
72.4G0.4 Ma and an isochron age of 72.8G0.4 (MSWDZ1.07). Unlike muscovite from the main phase of the Mid
Hills monzogranite, muscovite NY-49M is entirely of
igneous origin and helps to define the mylonitic foliation.
Muscovite (NY-2M) from the Mid Hills monzogranite in
Fig. 12. Summary of cooling histories determined from MDD modeling of K-feldspar. Grey pattern, 90% confidence intervals for the total distribution, and
black, 90% confidence for the median of the distribution (Lovera et al., 1997).
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1709
the footwall (Fig. 4) produced an age spectrum with an age
gradient from 66.3G0.8 to 71.7G0.6 Ma (Fig. 9c). Musco-
vite NY-2M, similar to other muscovite separates from the
Mid Hills monzogranite (NY-1M and NY-26M) occurs
dominantly as secondary (hydrothermal or metamorphic)
grains within microscopic fractures. Coexisting biotite (NY-
2B), of igneous origin, yields a discordant age spectrum
with an intermediate hump, with all heating steps (excluding
the first) between 72.5 and 74.7 Ma but no plateau, and a
total gas age of 72.8G0.4 Ma (Fig. 9c). The sample does not
yield a statistically valid isochron age. However, isochron
regression analysis suggests the presence of excess argon.
5.2. K-feldspar
K-feldspars were analyzed using detailed furnace step
heating including isothermal duplicates to obtain diffusion
properties (E, Do/r2) for application of the multiple diffusion
domain (MDD) modeling approach of Lovera et al. (1989,
1991). Calibration of the furnace was accomplished via a
double thermocouple experiment in which the actual sample
position (inner crucible) temperature and time history was
recorded during step heating cycles at different temperatures
(as measured by the outer control thermocouple). Inner
crucible temperatures and times for heating steps were used
in modeling. Activation energy (E) was determined using a
least squares linear regression of data from low-temperature
steps of the experiment plotted on an Arrhenius diagram
(Lovera et al., 1989). The frequency factor (Do) for each
diffusion domain was determined using the calculated
activation energy and modeling the Arrhenius plot (Lovera,
1992). Ten E–Do pairs were then randomly selected from a
Gaussian distribution around the values and their uncertain-
ties obtained from the Arrhenius diagram (Lovera et al.,
1997). For each pair, a single activation energy was
assumed to be representative of all domains used in the
modeling. The number of domains along with their size and
volume fraction was modeled using a variational iterative
technique to determine the best fit between the experimental
and modeled results on a domain size distribution plot
[log(r/ro) vs. %39Ar released] (Richter et al., 1991). Five
cooling histories were then determined for each E–Do pair
by fitting modeled age spectra to the experimental age
spectrum using these parameters and domain distributions.
The cooling histories obtained were then used to calculate
90% confidence intervals for the total distribution and the
median of the distribution (Lovera et al., 1997).
Two separates of K-feldspar at similar depths beneath the
Pinto shear zone (NY-26K and NY-2K, western and eastern
transects, respectively; Fig. 4), and two from the hanging
wall (NY-1K and NY-14K, western transect; Fig. 4) 500 m
and 2 km structurally above the top of the shear zone, were
analyzed and modeled to construct thermal histories; all
samples are from the Mid Hills monzogranite. The age
spectra for all K-feldspar are shown in Fig. 10; an example
of the modeled Arrhenius, domain distribution, age
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201710
spectrum, and cooling history plots are shown in Fig. 11 of
K-feldspar NY-2K; the complete modeling results for the
other three K-feldspars are available as an electronic
supplement in Appendix E; the resulting model cooling
histories are presented in Fig. 12. The Arrhenius and domain
distribution plots of the samples and the models produced
very good to excellent fits (Fig. 11), indicating that the
diffusion properties determined in the laboratory data
approximate those acting in the sample when in the natural
geological environment, and that the furnace temperature is
stable and well calibrated. The four K-feldspar age spectra
show very similar forms, with steep age gradients in the first
10–15% 39Ar released rolling over into shallow age
gradients in the remainder of the analyses (Fig. 11). The
K-feldspars in this study are not affected by excess argon, as
indicated by comparison between calculated K-feldspar
closure temperatures and muscovite and biotite cooling
ages, analysis of the argon isotopic ratios using inverse
isochrons, and the lack of a decrease in age in the second of
the isothermal duplicate heating steps. Therefore, the
K-feldspar samples are ideal for thermal modeling.
The activation energies determined for the four
K-feldspars range from 46.1G2.7 to 52.7G5.0 kcal/mol
for the hanging wall samples (NY-1 and NY-14, respect-
ively) and 58.7G5.8 to 50.5G2.4 kcal/mol for the footwall
samples (NY-26 and NY-2, respectively). The modeled
cooling curves for the two footwall K-feldspars define
average cooling rates of 62 8C/m.y. (NY-2K) and
76 8C/m.y. (NY-26K) from 71 to 68 Ma (Fig. 12). The
hanging wall K-feldspars produced modeled cooling curves
with cooling rates of 33 8C/m.y. (NY-1K) and 28 8C/m.y.
(NY-14) from 72 to 66 Ma.
5.3. Interpretation of mica age spectra
The stair-stepped age spectra of the muscovite from the
Mid Hills monzogranite (Fig. 9a–c and e) may be
interpreted as either slow (NY-1) or moderately slow
(NY-26M and NY-2M) cooling through the closure
temperature interval for muscovite, or a mixture of
radiogenic argon from hydrothermal and igneous musco-
vite, representing a mixing between growth and cooling
ages, respectively. We favor a mixed age origin for the age
gradients, with younger ages at lower experimental
temperatures corresponding to gas release from hydrother-
mal muscovite, because: (1) petrographic examination
shows the presence of both hydrothermal and igneous
muscovite, although in varying abundance between
samples; (2) the apparent ages for the lower furnace-
temperature portions of the age gradients are younger than
apparent ages for coexisting and adjacent biotite as well as
muscovite from alaskite and quartz veins within the shear
zone (Fig. 9b and c) and K-feldspar ages for domains with
closure temperatures similar to micas (w300–400 8C); (3)
apparent K–Ca ratios (Appendix C) of muscovite are more
highly variable for the samples from the Mid Hills
monzogranite (NY-1M, NY-2M, and NY-26M) than for
the quartz vein or alaskite (NY-25 and NY-49); (4) the
cooling rates determined from modeled retentive footwall
K-feldspar shows a relatively rapid, not slow, cooling rate of
62–76 8C/m.y. that encompasses a portion of the closure
temperature interval for muscovite; and (5) preservation of
growth ages for hydrothermal muscovite is consistent with
the inferred temperatures for fracturing. The fractured
K-feldspar in the shear zone is surrounded by plastically
deformed quartz, and fractures are filled with quartzCmuscovite, which implies that a component of deformation
occurred within the 400–250 8C temperature range (Hirth
and Tullis, 1992; FitzGerald and Stunitz, 1993; Pryer,
1993). If the muscovite growth in fractures in the shear zone
is coeval with muscovite growth within fractures in the
footwall and hanging wall, then it follows that footwall
muscovite growth occurred within the temperature range of
250–400 8C, consistent with the ages recording growth and
not cooling. Furthermore, the age gradient may predomi-
nantly reflect progressive growth of hydrothermal musco-
vite during deformation (e.g., Kirschner et al., 1996), with
the majority of radiogenic argon evolved from hydrothermal
grains. Hydrothermal muscovite growth occurred during
cooling and inferred exhumation, and if the latter
interpretation is correct, was apparently synchronous in
the two footwall sample localities, as indicated by the
similarity in morphology between the two footwall
muscovite age spectra (Fig. 9e).
In summary, the mica 40Ar/39Ar results are consistent
with 74–72 Ma cooling of igneous mica followed by 72–
69 Ma growth of hydrothermal muscovite during cooling.
Despite the differences in detail, the similarity in ages
indicate that cooling of igneous mica and growth of
muscovite in fractures occurred during a relatively short
time interval within the period of rapid cooling shown by
the K-feldspar MDD modeling.
6. Discussion
6.1. Does the Pinto shear zone record extension or
shortening?
Previous workers have speculated that the Pinto shear
zone is a Mesozoic thrust (Beckerman et al., 1982) or
alternatively a Late Cretaceous normal fault (Miller et al.,
1996). This discrepancy in part reflects the difficulty of
distinguishing extensional from contractional shear zones,
perhaps one of the foremost challenges in structural studies,
and in particular in deformed intrusive complexes that lack
useful stratigraphic or metamorphic relationships between
the hanging wall and footwall. To distinguish an extensional
from a contractional origin for the Pinto shear zone and
resolve the contrasting interpretations we bring to bear a
broad array of observations including shear zone geometry
and kinematics, hanging wall deformation style, progressive
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1711
changes in deformation temperature, and differences in
hanging wall and footwall thermal histories; we conclude
that the Pinto shear zone is a ductile normal fault.
6.1.1. Shear-zone geometry and kinematics
Shear-sense indicators within the Mid Hills monzogra-
nite consistently demonstrate top to-the-SSW, generally
down-dip shear sense consistent with normal-sense motion
in the present geographic coordinates. The foliation in the
shear zone varies in its dip from 20 to 658SW. The
possibility of tilting (to the SW) a NE-dipping thrust fault by
more than 658 is considered unlikely, as suggested by
geologic relations in the Live Oak Canyon area, 7 km to the
NE (Fig. 4). The Live Oak Canyon granodiorite intrudes
into the base of a Cretaceous volcanic succession,
concordant to the overlying low-angle Sagamore Canyon
thrust, which it deforms into a structural dome (Burchfiel
and Davis, 1977; Smith et al., 2003). These relationships are
most consistent with a position in the roof of a pluton
(laccolith) rather than a pluton sidewall, suggesting no large
magnitude post-intrusion tilt.
6.1.2. Hanging wall kinematics
The geometric and kinematic relationships between top-
to-the-SW shearing within the main shear zone and
antithetic high-angle shears localized at dike margins within
the hanging wall are most compatible with extensional
rather than contractional motion along the Pinto shear zone.
The antithetic high-angle shears extend the hanging wall
and merge downward with the top of the main shear zone,
and are interpreted as developing synchronously with the
main shear zone (Fig. 7). Normal faults that merge
downwards with detachment faults and occur in their
hanging walls are very common in metamorphic core
complexes and detachment fault systems (e.g. Crittenden et
al., 1980; Stewart and Argent, 2000). Although normal
faults that developed coevally with and soling into major
thrust zones have been documented (Yin and Kelty, 1991),
these examples are few and are associated with contrac-
tional structures such as minor thrusts, folds and cleavage
and may record a part of the non-coaxial strain field in the
base of the thrust sheet. No contractional structures have
been documented either in the footwall or hanging wall of
the Pinto shear zone that postdate the intrusion of the Mid
Hills monzogranite and interact with the shear zone, or that
postdate the Cretaceous phases of the Teutonia batholith in
the larger region (Fig. 3).
6.1.3. General-shear thinning of shear zone
In addition to thinning of the hanging wall, the main
shear zone was also thinned during deformation as shown by
the synthetic rotation of antithetically sheared N-dipping
porphyry dikes (Fig. 7). Thus, the bulk flow within the shear
zone was general shear (e.g. Simpson and DePaor, 1993) in
which the coaxial component accomplished shortening
perpendicular to the shear-zone boundary and extension
parallel to the boundary and in the shear direction. The
folding of dikes that cross foliation at a high angle and strike
parallel to the transport direction, about axes parallel to the
shear direction, also record shortening perpendicular to the
shear-zone boundary. While thinning shear zone behavior is
not diagnostic of ductile normal faults as opposed to thrusts,
vorticity studies of extensional shear zones commonly show
such thinning shear-zone behavior (Lee et al., 1987; Wallis,
1992; McGrew, 1993; Wells, 2001; Bailey and Eyster,
2003).
6.1.4. Footwall and hanging wall thermal histories
Thermal modeling and thermochronometric studies have
shown that the hanging walls and footwalls to both thrust
and normal faults experience markedly different thermal
histories. One- and two-dimensional thermal models of
normal faults suggest that footwalls experience rapid
cooling rates during and following rapid faulting (Ruppel
et al., 1988; Grasemann and Mancktelow, 1993; Ketcham,
1996); rapid cooling documented by numerous thermo-
chronometric studies of the footwalls of detachment faults
fit these models well (e.g. Hoisch et al., 1997; Foster and
John, 1999; Wells et al., 2000). The thermal history of
hanging walls to detachment faults have received less
attention, but several one- and two-dimensional thermal
modeling studies show that hanging walls may experience
heating due to juxtaposition against a hot footwall if the
advection of heat in the footwall is sufficient due to rapid
displacement (Grasemann and Mancktelow, 1993; Dunkl
et al., 1998). Cooling, as the geothermal gradient
reequilibrates, is predicted to follow reheating after the
cessation of slip. In contrast, the footwalls to thrusts should
experience heating during or following thrusting with peak
temperatures following the cessation of thrusting (England
and Thompson, 1984; Ruppel and Hodges, 1994), whereas
hanging walls should experience cooling during erosional
denudation concurrent with rock uplift and slip.
In the construction of cooling histories for the footwall
and hanging wall of the Pinto shear zone, we have not
explicitly integrated the mica 40Ar/39Ar results but rather
have relied upon the MDD modeling of the K-feldspar argon
data. The K-feldspar MDD modeling procedure is robust in
extracting meaningful cooling histories from well-behaved
K-feldspars such as those from the Mid Hills monzogranite,
as the diffusion parameters for each individual K-feldspar
separate can be measured directly and the uncertainty in
estimation of these parameters can be propagated into the
determination of cooling history (Lovera et al., 1997;
Fig. 12). The analyzed K-feldspars were relatively retentive
to argon diffusion, resulting in cooling histories that reached
or spanned the commonly assumed ‘nominal’ closure
temperatures for muscovite and biotite (McDougall and
Harrison, 1999). The uncertainties in estimating closure
temperatures for the micas of this study are significant
resulting from a number of factors including the difficulty in
extracting the kinetic parameters of diffusion from hydrous
Fig. 13. Synoptic diagram of constraints on the timing of deformation
within the Pinto shear zone. Dashed grey lines depict two of many possible
pre-extensional cooling histories for the 90 Ma Mid Hills monzogranite,
differing in depth of emplacement and extent of Late Cretaceous heating.
Cooling paths determined from K-feldspar MDD modeling are shown.
Open boxes indicate U–Pb zircon ages. Microstructures suggest defor-
mation began at w500 8C and continued to cataclastic conditions at
!250 8C.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201712
minerals, potential compositional control on argon diffusion
and uncertainties in the effective diffusion radius (McDougall
and Harrison, 1999, and references therein). Furthermore,
there is petrographic and isotopic evidence for two populations
of muscovite in the samples from the Mid Hills monzogranite.
It should be noted, however, that for samples exhibiting a
single mica population (NY-40B and NY-49M), by assuming
that the physical grain size represents the effective diffusion
dimension and using the fast cooling rates as determined by the
MDD modeling of K-feldspar, calculated closure tempera-
tures are significantly higher than ‘nominal’ temperatures,
making their placement in T–t space consistent with the
K-feldspar MDD modeled cooling histories.
The difference in the cooling rates and positions of the
cooling paths for the hanging wall and footwall of the Pinto
shear zone supports an extensional interpretation (Fig. 13).
The two footwall samples cooled at rates of 62 and 76 8C/
m.y. whereas the two hanging wall samples cooled at about
50% of those rates, 28 and 33 8C/m.y. The cause of cooling
of the hanging wall will be discussed further in Section 6.3.
6.1.5. Deformation during decreasing temperature
conditions
Microstructural studies indicate that the bulk of the
distributed deformation occurred at upper and middle
greenschist facies conditions, with temperatures decreasing
during deformation, as shown by kinematically compatible
brittle deformation overprints on early plastic deformational
features—in particular for feldspar. This superposition of
lower temperature on higher temperature deformation is
interpreted to have occurred during the progressive
unroofing of the footwall block. Such decreasing tempera-
ture conditions during progressive shearing are commonly
observed in extensional shear zones (Lister and Davis,
1989) and are predicted by thermal models (Grasemann and
Mancktelow, 1993; Ketcham, 1996).
The new kinematic and deformation-mechanism studies
presented here provide a compelling case in support of the
reinterpretation of the Pinto shear zone as an extensional
shear zone (Miller et al., 1996), which requires revision in
the existing correlation of faults across the Ivanpah Valley.
The Pinto shear zone was previously interpreted as a thrust
fault (New York Mountain thrust of Beckerman et al.
(1982)) and correlated across the Ivanpah Valley to the
Morning Star thrust (Burchfiel and Davis, 1971) of the
southern Ivanpah Mountains (Fig. 3). Subsequent kinematic
studies of the west-dipping Morning Star thrust have
confirmed thrust-sense, top-to-the-E kinematics (Sheets,
1996). The recognition that the Pinto shear zone records top-
to-the-SW normal-sense motion, rather than top-to-the-E
thrusting, precludes the correlation of these faults. Below
we more explicitly discuss the geochronologic and
thermochonologic data that constrains the Pinto shear
zone as Late Cretaceous in age.
6.2. Age of the Pinto shear zone
The timing of motion along the Pinto shear zone is
well constrained by the combination of U–Pb crystal-
lization ages (Smith et al., 2003), the thermal history of
footwall rocks derived from 40Ar/39Ar thermochronology,
and microstructural studies. The shear zone deforms
74.6G3.2 Ma (U–Pb, zircon) porphyry dikes, which
provides a maximum age on deformation; however, the
dikes may have intruded early in the extensional history.
Microstructural studies indicate that the bulk of the
distributed deformation occurred at upper and middle
greenschist-facies conditions, with temperatures decreas-
ing during deformation down to cataclastic conditions.
The cooling history of the footwall is well defined by the
MDD modeling results of two K-feldspars, which shows
rapid cooling from 420 to 220 8C over the time interval
71–68 Ma. Consideration of the deformation tempera-
tures, in conjunction with the cooling history, indicates
that much of the ductile fabric was acquired between O71
and 68 Ma (Fig. 13). An abrupt reduction in footwall
cooling rates at 67–68 Ma is coincident with a conver-
gence in the cooling paths for the footwall and hanging
wall, and is interpreted to indicate the end of cooling
resulting from exhumation by the Pinto shear zone
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1713
(Figs. 12 and 13). This reduction in cooling rate may
postdate the actual reduction or termination of slip due to
the time lag for thermal re-equilibration of the advected
isotherms (Ketchum, 1996). An alternative to the apparent
reduction in cooling evident from the modeling, which
assumed monotonic cooling, is that the age gradient in the
low-T portion of the K-feldspar age spectra results from a
moderate reheating. However, Paleocene to early Eocene
reheating in this region has not been recognized.
Constraining the age of initiation of extension is more
difficult without higher temperature thermochronology
(e.g. hornblende) or a more accurate constraint on the age
of the dikes. Extrapolating the 62–76 8C/m.y. footwall
cooling rate to the higher temperatures present during
initial shearing, as inferred by the K-feldspar plasticity
and myrmekite formation, suggests early higher tempera-
ture shear initiated before w72 Ma (Fig. 13). Therefore,
extensional motion on the shear zone initiated between 78
(maximum age of dike; Fig. 13) and 72 Ma, and was over
by 68 Ma.
6.3. Mechanisms to explain hanging wall cooling
An unexpected result of this study is the relatively rapid
cooling rate determined for the hanging wall and the lack of
significant discordance in mica ages between hanging wall
and footwall. Below we discuss several possible expla-
nations including: (1) refrigeration of the lithosphere by
shallow Laramide subduction; (2) heating of the hanging
wall by the hot footwall, and subsequent cooling,
sympathetic but slower than in the footwall; (3) cooling of
the hanging wall due to an unrecognized structurally higher
coeval normal fault; and (4) cooling following heating
resulting from intrusion of a pluton at depth.
Dumitru et al. (1991) proposed that the shift from normal
to shallow subduction during the Laramide orogeny caused
widespread refrigeration of the North American lithosphere
in the western Cordillera, including the eastern Mojave
Desert region. During low-angle subduction, the western
Cordillera may have developed a much colder, forearc-like
thermal structure due to refrigeration of the lithosphere by
the relatively cool underlying subducting slab that displaced
hotter mantle asthenosphere. The refrigeration effect is
hypothesized to have migrated eastward through time along
with the eastward propagating low-angle slab, with
refrigeration in the eastern Mojave Desert region (Old
Woman and Chemehuevi Mountains) beginning ca. 68 Ma
(fig. 4 of Dumitru et al., 1991). This shift from an arc-like to
forearc-like geotherm could have important thermal and
rheological effects in the western Cordillera, including
strengthening and increased resistance to gravitational
collapse. Refrigeration and strengthening coeval with
extension thus seems implausible. Therefore, it is unlikely
that the rapid cooling evident in the hanging wall and
footwall of the Pinto shear zone, which was synchronous
with extensional shearing along the zone, is related to
Laramide refrigeration. However, refrigeration is a viable
explanation for post-68 Ma cooling, after the convergence
of cooling paths for the hanging wall and footwall (Fig. 13),
and may have contributed to the cessation of Late
Cretaceous extension.
Alternatively, cooling of the hanging wall may record
thermal relaxation following heating due to juxtaposition
against a hot footwall. Such reheating, followed by cooling,
is predicted by two-dimensional thermal modeling and has
been documented by low-temperature thermochronology
and thermal-maturation studies of the hanging wall to the
Rechnitz Window of the eastern Alps (Dunkl et al., 1998).
The similarity of the footwall mica ages and the mica ages
from the base of the hanging wall, 500 m above the shear
zone, supports the interpretation that the hanging wall was
heated by juxtaposition against a hot footwall, perhaps aided
by fluid advection during shearing as suggested by wide-
spread secondary mineralization of muscovite and quartz in
the hanging wall, footwall, and within the shear zone. Such
heating would be followed by relatively rapid cooling of the
hanging wall, as the geothermal gradient relaxed following
the cessation of slip. This mechanism for hanging wall
heating followed by cooling may explain the difference in
modeled K-feldspar cooling histories between the samples
500 m and 2 km from the fault, but cannot explain the
cooling evident 2 km above the fault (NY-14). Two-
dimensional finite difference thermal modeling indicates
that the thermal anomaly across the fault is not of sufficient
magnitude to provide significant heat conduction across
2 km of the hanging wall; therefore, an additional
mechanism is needed to explain the thermal history of
NY-14.
Cooling of the hanging wall may be caused by tectonic
denudation along a now exhumed and eroded structurally
higher normal fault that once projected above the study area.
Although no such structures are present between the shear
zone and the Cenozoic alluvium of Ivanpah Valley, an
unrecognized extensional structure may exist further to the
west. Late Cretaceous cooling of mid-crustal granites in the
Granite Mountains to the southwest (Fig. 2) (Miller et al.,
1996; Kula et al., 2002) requires an unrecognized
exhumational structure of similar age to the Pinto shear
zone; this structure may have projected above the New York
Mountains.
Finally, a portion of the Late Cretaceous cooling may
result from cooling, following heating, resulting from
intrusion of a large Cretaceous pluton at depth. The
widespread occurrence of 74–80 Ma dikes in the area
allows this possibility. Such conductive cooling, following
heating in the roof of a pluton would allow for the relatively
rapid cooling rates seen in the hanging wall to the shear
zone.
To conclusively distinguish between the remaining
potential causes for cooling of the hanging wall block—a
structurally higher normal fault, thermal effects following
juxtaposition against a hot footwall, or conductive cooling
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201714
in the roof of a pluton—additional thermochronological
studies, coupled with thermal modeling, from a variety of
structural levels within the hanging wall and from the larger
region are required. However, despite the uncertainty in
explanation of cooling of the hanging wall, the fundamental
conclusion of this study is not diminished—a large
component of shearing within the Pinto shear zone occurred
during rapid cooling in the Late Cretaceous.
6.4. The cause of Late Cretaceous extension and peralu-
minous magmatism in the eastern Mojave
The established Late Cretaceous age and extensional
origin for the Pinto shear zone allows its integration with
other structures of similar age and kinematic significance in
the eastern Mojave Desert and southern Great Basin, to
provide a framework for evaluating the cause for initiation
of synconvergent extension in the southwestern Cordillera.
Hodges and Walker (1992) reviewed evidence for early
Cretaceous to Paleocene extension within the interior of the
Cordilleran orogen, from northern Washington to southern
California. Subsequent studies, outlined below, in the
eastern Mojave Desert have confirmed the importance of
Cretaceous extension in the evolution of the orogen and
allow for a better definition of its age and continuity.
An extensional origin and Late Cretaceous age for
mylonitic gneiss in the roof zone of the Cadiz Valley
batholith in the Iron Mountains (Fig. 2) (Miller and Howard;
1985) has been established (Wells et al., 2002). A
systematic decrease in biotite 40Ar/39Ar cooling ages in
the transport direction beneath a O1.3 km thick stack of
top-to-the-E mylonitic rocks indicates extensional shearing
bracketed between crystallization of porphyritic monzo-
granite (75G2, U–Pb zircon) and muscovite and biotite40Ar/39Ar ages of 67–69 Ma (Wells et al., 2002). Con-
cordant magmatic and solid-state foliations and synmag-
matic shearing fabrics suggest synextensional pluton
emplacement. Similar timing for extensional shearing and
intrusion is recorded in the Old Woman Mountains (Fig. 2).
Synmagmatic deformation within the Old Woman pluton
(74G3 Ma; Foster et al., 1989), and solid-state mylonitic
deformation within the pluton and wall rock screens, record
wE–W extension and vertical shortening (McCaffrey et al.,
1999). When combined with earlier studies, including those
of the western Old Woman Mountain shear zone, it is
apparent that extension was active from intrusion at 74 Ma
to 40Ar/39Ar muscovite closure at 68 Ma (Carl et al., 1991;
Foster et al., 1992; McCaffrey et al., 1999). Rapid
exhumation of mid-crustal rocks in the Granite Mountains
of the Mojave National Preserve (Fig. 2) has been
documented by barometry, geochronology, and thermo-
mochronology of 75–76 Ma granites. Al-in hornblende
geobarometry indicates intrusion at w4.5 kbar followed by
rapid cooling through hornblende 40Ar/39Ar closure inter-
preted as rapid conductive cooling to country rock
temperatures (Kula, 2002; Kula et al., 2002). Continued
rapid cooling from 350 to 60 8C from 73 to 68 Ma, as shown
by K-feldspar MDD and apatite fission track-length
modeling, is interpreted as resulting from tectonic exhuma-
tion (Kula et al., 2002). Metamorphic rocks in the Funeral
Mountains of the Death Valley region (Fig. 1) were
exhumed along the Cenozoic Boundary Canyon detachment
fault (Wright and Troxel, 1993; Hoisch and Simpson, 1993);
however, an earlier history of partial exhumation along the
Monarch Canyon and Chloride cliff shear zones has been
suggested (Applegate et al., 1992; Applegate and Hodges,
1995). The age of deformation is constrained by U–Pb
dating of synkinematic pegmatite (72G1 Ma, zircon) and
postkinematic pegmatite (70G1 Ma, zircon) (Applegate et
al., 1992). Extension of this age may be more widespread in
the Death Valley region: a shear zone in the Panamint
Range at an equivalent stratigraphic position to the Chloride
Cliff shear zone, the Harrisburg fault (Hodges et al., 1990;
Andrew, 2001), may also be Late Cretaceous. Based on
these examples and the constraints provided by the Pinto
shear zone, extension was apparently widespread in the
narrow time interval between 75 and 68 Ma along the axis
of the southwestern Cordilleran orogen.
These sites of Late Cretaceous extension all have early-
to syn-extensional plutons or dikes that are part of the belt of
Late Cretaceous granites of the Cordilleran interior (Figs. 1
and 2), dominantly of strongly peraluminous compositions
(Miller and Barton, 1990; Barton, 1990; ‘Cordilleran
peraluminous granites’ of Patino Douce (1999)). These
rocks have isotopic signatures consistent with variable
sources in Precambrian continental basement and are widely
attributed to be products of crustal anatexis (Farmer and
DePaolo, 1983; Miller, 1985; Patino Douce et al., 1990;
Wright and Wooden, 1991). A mantle component to these
magmas has also been suggested as experimental studies
implicate hybridization of basaltic melts through interaction
with Precambrian metagreywacke (Patino Douce, 1999),
and field observations, although uncommon, provide
evidence for interaction between felsic and mafic magmas
(Foster and Hyndman, 1990; Kapp et al., 2002). These
granites are largely restricted to areas underlain by
Precambrian basement that has undergone Mesozoic short-
ening. While crustal thickening was certainly a necessary
precondition for melting (Patino Douce et al., 1990),
additional factors required for anatexis are controversial,
including: fluid infiltration into hot crust from a shallow
Laramide slab (Hoisch, 1987; Hoisch and Hamilton, 1990),
increased mantle heat flux (Armstrong, 1982; Barton, 1990),
heating through mafic magmatic underplating or intrusion,
or decompression melting (Hodges and Walker, 1992;
Harris and Massey, 1994). The common association
between peraluminous plutons and extension is suggestive
of either a causal relationship or a shared root cause.
Decompression melting is excluded for a number of reasons
(Barton, 1990) including the observation here that extension
is syn-emplacement to post-emplacement, but not pre-
emplacement to plutons.
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1715
Synconvergent extension, in theory, may have been
driven by either a reduction in horizontal compressive stress
transmitted across the Farallon–North American plate
boundary and/or basal decollement to the Cordilleran
orogen, or an increase in forces resulting from lateral
contrasts in gravitational potential energy. Because the
Farallon–North American plate relative convergent velocity
increased in the Late Cretaceous (Engebretson et al., 1985),
as did the predicted buoyancy of the subducted slab
(Henderson et al., 1984), synconvergent extension due to
slab rollback (Royden, 1993) is precluded. We see two
explanations for synorogenic extension, although not
mutually exclusive, as compatible with the observations
from the southwestern Cordillera: (1) decoupling of the
middle to upper crust from the mantle lithosphere by
development of a low-viscosity lower crust, allowing lateral
contrasts in gravitational potential energy to relax by
extensional flow, and (2) removal of mantle lithosphere,
thereby increasing the lateral contrast in potential energy.
Widespread melting and attendant lowering of the
viscosity of the lower crust, required in the production of
Cordilleran peraluminous granites, may have played a role
in initiating synconvergent extension in the southwest
Cordillera. Experimental petrology and petrochemical
Fig. 14. Tectonic cartoon for Late Cretaceous removal of mantle lithosphere. A s
mass balance (strain compatibility) requires shortening of mantle lithosphere equiv
backarc. Instability of thickened mantle lithosphere leads to delamination causing:
from basalt intrusion and conductive heating of thinned lithosphere, crustal me
extension.
modeling of the peraluminous granites require a deep
crustal source (Patino Douce, 1999; Kapp et al., 2002). The
site of melting was most probably diffuse and sheet-like as it
was controlled by the thermal and lithologic structure of the
lithosphere (Sawyer, 1998). A reduction in strength during
partial melting of the lower crust has the effect of
decoupling the overlying crust from the underlying mantle,
effectively insulating the crust from stresses due to plate
coupling, thus removing the lateral support and allowing the
overlying crust to respond to lateral gradients in gravita-
tional potential energy (Vanderhaeghe and Teyssier, 2001).
If extension occurred after the low-angle slab made contact
with eastern Mojave continental lithosphere, fluid infiltra-
tion from a dewatering Farallon slab could have further
weakened the lower crust (Malin et al., 1995).
Widespread Late Cretaceous heating, anatexis, magma-
tism, and extension may have been promoted by removal of
mantle lithosphere prior to 75 Ma, either prior to the arrival
of a flat slab beneath the eastern Mojave region or during its
initial impingement (Fig. 14). The removal of lithospheric
mantle by delamination (Bird, 1979) or convection
(Houseman et al., 1981; England and Houseman, 1988)
has been proposed as an effective mechanism to dramati-
cally increase the gravitational potential energy of a
ignificant root of mantle lithosphere developed due to Sevier orogenesis—
alent in magnitude to shortening of crust in fold–thrust belts of the Mesozoic
adiabatic melting of asthenosphere, transfer of heat into lower crust resulting
lting and magmatism, buoyancy-driven uplift, and gravitationally-driven
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–17201716
mountain belt and promote surface uplift and horizontal
extension. Removal of lithospheric mantle results in an
increase in Moho temperature and geothermal gradient, and
may produce mafic magmatism through adiabatic decom-
pression of asthenosphere (Platt and England, 1993; Kay
and Kay, 1993; Kay and Abbruzzi, 1996). This process
would provide the additional heat necessary for crustal
anatexis, and heat transfer could be rapid if by basaltic
intrusions (Leventhal et al., 1995; Annen and Sparks, 2002).
We are not explicit in addressing the mechanism by which
the lithosphere may have been removed during the onset of
the Laramide orogeny. Removal of mantle lithosphere,
whether by delamination (Bird, 1979), convective removal
(England and Houseman, 1988) or viscous ablation,
produces a similar response of heating and buoyancy of
the remaining lithosphere, and thus a first order under-
standing of the responses (i.e. extension, crustal melting) is
not reliant on the details of the mechanism of removal.
Delamination and lower crustal weakening are not
necessarily separate and alternative mechanisms, but may
have worked together to create the rheological and dynamic
state necessary to extend the crust.
Delamination is proposed here to have occurred
immediately before eastward propagation of low-angle
subduction of the Farallon plate, during the inception of
the Laramide orogeny. The recognition of a thin and fertile
Archean mantle lithosphere beneath the eastern Mojave
from xenolith studes (Lee et al., 2001) shows a dense mantle
lithosphere capable of density-driven foundering and
thickness compatible with prior removal. Removal of
lithosphere was required to allow flattening of the slab to
achieve a low-angle geometry, and we suggest that
delamination, whether piecemeal or involving larger blocks
of foundered lithospheric mantle, took place before the
asthenospheric mantle wedge was expelled. The presence of
asthenospheric mantle wedge beneath the eastern Mojave
prior to 75 Ma allows displacement of detached lithosphere
via counterflow, and upwelling of asthenosphere to produce
basaltic melts (Leventhal et al., 1995). Lithosphere removal
by delamination, rather than subduction-erosion, allows the
underplating of water-rich metasediments, as evident in the
tectonic windows of the Rand–Pelona–Orocopia schist belt
(Jacobsen et al., 1996). Furthermore, the presence of
asthenospheric mantle during lithosphere removal allows
the North American lithosphere to be isostatically indepen-
dent of the load of the Farallon slab, rather than isostatically
coupled, and to uplift rather than subside (Cross and Pilger,
1978; Bird, 1984). Delamination or detachment of litho-
spheric mantle explains many enigmatic yet prevalent
aspects of the metamorphic, magmatic, and kinematic
history of the southwestern Cordilleran orogen.
7. Conclusions
The following observations are used to establish that the
Pinto shear zone is a Late Cretaceous extensional shear zone
rather than a ductile thrust. (1) Foliation in the Pinto shear
zone generally dips 20–658S and SW with lineations
plunging to the SSW, and kinematic studies consistently
demonstrate top-to-the-SSW shearing. (2) The hanging wall
of the Pinto shear zone, and low-strain domains within the
shear zone, were extended by synthetic rotation of blocks
bound by antithetic north-dipping normal-sense shear zones
localized along the margins of or within porphyry dikes.
Structural relationships between these antithetic shear zones
and the main shear zone indicate simultaneous displace-
ment. (3) Differential cooling rates are evident between
hanging wall and footwall. Cooling rates constrained by
K-feldspar 40Ar/39Ar MDD modeling are 62–76 8C/m.y. for
the footwall and 28–33 8C/m.y. for the hanging wall. (4)
Microstructural studies indicate deformation during
decreasing temperatures, from lower amphibolite–upper
greenschist facies to cataclastic conditions, interpreted to
record progressive unroofing of the footwall block during an
enigmatic ‘regional’ cooling as evident by moderate cooling
rates of the hanging wall.
The timing of extension is well constrained by the
combination of a U–Pb crystallization age on deformed
porphyry dikes and integration of estimated deformation
temperatures with the dated thermal history. Deformation of
74.6G3.2 Ma porphyry dikes together with a reduction in
cooling rate at 67–68 Ma, marking the termination of rapid
cooling related to tectonic denudation, brackets extension
between !78 and 68 Ma. The cooling history of the
hanging wall close to the shear zone (500 m) and similarity
between ages of hydrothermal muscovite in the footwall and
hanging wall suggest conductive heating of the hanging wall
by the hot footwall aided by fluid advection, and consequent
sympathetic but slower cooling than the footwall. However,
the relatively rapid cooling experienced by the hanging wall
at greater distances from the shear zone (w2 km) requires
an additional mechanism for cooling; either an unrecog-
nized structurally higher normal fault or a Late Cretaceous
pluton at depth, are permissive.
Motion along the Pinto shear zone is contemporaneous
with other extensional structures, regional exhumation and
cooling of mid-crustal rocks, in the eastern Mojave Desert.
We contend that the common 74–68 Ma cooling ages in the
Mojave Desert region may in many cases record post-
intrusive cooling and exhumation by extensional structures.
Refrigeration of the Cordilleran lithosphere (Dumitru et al.,
1991) or erosional denudation (George and Dokka, 1994)
may be locally important, and refrigeration should postdate
cooling related to extension and erosion. Late Cretaceous
extension at 75–68 Ma was regional in scale in the
southwestern Cordillera, and therefore requires a regional
causative mechanism. We propose that widespread Late
Cretaceous crustal melting and magmatism followed by
extension and cooling in the Mojave sector of the
Cordilleran orogen is most consistent with production of a
low-viscosity lower crust during anatexis and/or removal of
M.L. Wells et al. / Journal of Structural Geology 27 (2005) 1697–1720 1717
mantle lithosphere at the onset of Laramide shallow
subduction. From the perspective of the Sevier and
Laramide orogens as a composite contractional orogen,
the extension and magmatism in the Mojave Desert region
was synchronous with continued convergence between the
Farallon and North America plates, and continued short-
ening in the Sevier fold–thrust belt and Laramide province
to the north and in the Laramide province to the south.
Acknowledgements
Financial support for this research was provided by NSF
Grant EAR 96-28540 awarded to MLW, and grants from the
Geological Society of America and UNLV Department of
Geoscience to MAB. The Nevada Isotope Geochronology
Laboratory was funded by NSF Grant EPS-9720162 to TLS.
We have benefited from discussions of Mojave geology
with D. Foster, M. Grove, K. Howard, E. Humphreys, and
C. Jacobson. R. Allmendinger is thanked for use of
Stereonet and FaultKin. K.A. Howard and A.J. McGrew
provided thorough and insightful reviews.
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